The present invention relates to cemented carbide rolls for hot rolling of steel wire and rod. These rolls are made from cemented carbide grades containing WC and a binder phase of either Co or an alloy of Co+Ni or Co+Ni+Cr.
One of the main advantages of using cemented carbide for rolls for hot rolling compared to rolls made of cast iron, steel or high speed steel, is the lower surface temperature of the cemented carbide roll as a result of the excellent heat conductivity of cemented carbide. This delays the initiation of thermal cracks and decreases the abrasive wear in the passform which is the groove shaped part of the roll which comes in contact with the hot stock. It will also reduce the fatigue from the thermal cycling of the roll. Altogether, cemented carbide rolls often have a passform life 10-20 times that of rolls made of other materials. This has lead to a widespread use of cemented carbide rolls for hot rolling of wire, bar and profiles.
The thermal conductivity of cemented carbide is inversely proportional to the content of binder phase. This is due to the higher thermal conductivity of tungsten carbide compared to the binder phase. When the binder phase content is increased, more heat transportation takes place in the binder phase due to reduced carbide/carbide interface area.
When choosing a composition for certain hot roll applications, it is often a balancing of the need for a tough material withstanding the mechanical stress with the need to minimize the binder phase content to get a material with as high heat conductivity as possible to withstand the formation of thermal cracks and thermal fatigue and to get as long passform life as possible without increasing the risk of cracking due to mechanical overload.
The high mechanical stress with a lot of blows from the cold ends of the hot stock being fed into the roll and high separating forces, has lead to the use of cemented carbide grades with hardness values ranging from 600 to 1250 HV.sub.3 and cobalt contents from 10% to 30% by weight. In order to maintain such low hardness values, it has been necessary to use as coarse grained grades as possible, to be able to reduce the binder phase content without increasing the hardness and thus reducing the toughness of the material.
Cemented carbide is made by powder metallurgical methods comprising wet milling a powder mixture containing powders forming the hard constituents and binder phase, drying the milled mixture to a powder with good flow properties, pressing the dried powder to bodies of desired shape and finally sintering.
The intensive milling operation is performed in mills of different sizes using cemented carbide milling bodies. Milling is considered necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is believed that the intensive milling creates a reactivity of the mixture which further promotes the formation of a dense structure during sintering. The milling time is in the order of several hours up to days.
The microstructure after sintering of a material manufactured from a milled powder is characterized by sharp angular WC grains with a rather wide WC grain size distribution, often with relatively large grains, which is a result of dissolution of fine grains, recrystallization and grain growth during the sintering cycle.
In U.S. Pat. Nos. 5,505,902 and 5,529,804, methods of making cemented carbide are disclosed according to which the milling is essentially excluded. Instead, in order to obtain a uniform distribution of the binder phase in the powder mixture, the hard constituent grains are precoated with the binder phase, the mixture is further mixed with a pressing agent, pressed and sintered. In the first mentioned patent, the coating is made by a SOL-GEL method and in the second, a polyol is used. When using these methods, it is possible to maintain the same grain size and shape as before sintering due to the absence of grain growth during sintering.